when I blew my HG, there was tons of white smoke coming out the exhaust (this is from the coolant/water evaporating that is in the oil).
also, look around the exhaust manifold, where the 5th and 6th cylinders are located, this is where the majority of HG fail on supras. sometimes there is coolant leaking out towards the outside. you might not be able to see this easily, so run the car around the block a couple of times then while running look at the engine and rev the engine by hand.
as stated previously, look for frothy coolant and frothy oil on the dipstick (goes with out saying, but dont open the radiator cap while hot).
the engine temperature gauge should be going buck wild, spiking up and down while driving, especially while idling.
good luck man